Positive active material for a rechargeable lithium battery, method for preparing the same and battery containing the same

ABSTRACT

A positive active material for a rechargeable lithium battery is provided. The positive active material comprises a lithiated intercalation compound and a coating layer formed on the lithiated intercalation compound. The coating layer comprises a solid-solution compound and an oxide compound having at least two coating elements, the oxide compound represented by the following Formula 1:
 
M p M′ q O r   (1)
 
wherein M and M′ are not the same and are each independently at least one element selected from the group consisting of Zr, Al, Na, K, Mg, Ca, Sr, Ni, Co, Ti, Sn, Mn, Cr, Fe, and V; 0&lt;p&lt;1; 0&lt;q&lt;1; and 1&lt; r ≦2, where r is determined based upon p and q. The solid-solution compound is prepared by reacting the lithiated intercalation compound with the oxide compound. The coating layer has a fracture toughness of at least 3.5 MPam 1/2 . A method of making the positive active material is also provided.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority of Korea Patent Application No.2001-65805 filed on Oct. 24, 2001 in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a positive active material for arechargeable lithium battery and a method for preparing the same, andmore particularly, to a positive active material for a rechargeablelithium battery having structural stability and improved cycle-lifecharacteristics and a method for preparing the same.

(b) Description of the Related Art

A rechargeable lithium battery having an average discharge potential ofapproximately 3.7 V, i.e. a battery having substantially 4V, isconsidered to be one of the essential components in the digitalgeneration since it is an indispensable energy source for portabledigital devices such as cellular phones, notebook computers, andcamcorders, which are often called “3C” devices.

A rechargeable lithium battery uses materials from or into which lithiumions are deintercalated or intercalated for positive and negative activematerials. For the electrolyte, a lithium salt solution in an organicsolvent or a polymer is used. A rechargeable lithium battery produceselectric energy as a result of changes in the chemical potentials of theactive materials during the intercalation and deintercalation reactionsof the lithium ions.

For the negative active material in the rechargeable lithium battery,metallic lithium was used in the early days of development. Recently,however, because the metallic lithium causes a short battery life due toits high reactivity with the electrolyte and the formation of lithiumdendrites, carbonaceous materials such as amorphous carbon orcrystalline carbon, which reversibly intercalate lithium ions, haveextensively been used instead of the metallic lithium. It has also beensuggested to add additives such as boron to the carbonaceous material inorder to improve the capability of carbonaceous material. For example, aboron-coated graphite (BOC) improves the performance characteristics ofthe carbonaceous materials.

For the positive active material in the rechargeable lithium battery,chalcogenide compounds into or from which lithium ions are intercalatedor deintercalated are used. Typical examples thereof include LiCoO₂,LiMn₂O₄, LiNiO₂, LiNi_(1-x)Co_(x)O₂ (0<x<1), and LiMnO₂. Amanganese-based positive active material such as LiMn₂O₄ or LiMnO₂ isattractive since it is readily prepared, is less expensive than theother materials, and is environmentally friendly. However, themanganese-based materials have the disadvantage a relatively lowcapacity. LiNiO₂ is inexpensive and has a high capability, but it isdifficult to prepare in the desired structure, and it becomesstructurally unstable during the charge and discharge. Among thesematerials, LiCoO₂ is most accepted in the battery market since itsoverall performance characteristics are better than the others.Accordingly, most of the current commercially available rechargeablelithium batteries (approximately 95%) adopt LiCoO₂ as the positiveactive material, but it is rather expensive. There is a great deal ofeffort being expended to find an alternative, in order to reduce thecost of the active material.

The positive active material for the rechargeable lithium battery isalso called a Li-intercalation compound because its structural stabilityand capacity are determined by the nature of reversibleintercalation/deintercalation reactions of lithium ion. The structure ofthe Li-intercalation compound is converted during theintercalation/deintercalation reaction of lithium ion, and itsstructural stability is strongly influenced by the composition of thepositive active material, Li_(x)MO₂ (M═Ni or Co), i.e. the value of x.For example, when x is at least 0.5, the phase transition occurs from ahexagonal phase to a monoclinic phase, while when x is less than 0.5,the hexagonal phase reappears. Anisotropic volumetric expansion causedby the phase transition will generate micro-cracks on the positiveactive material, causing damage to its morphological structure, andcharge-discharge efficiencies of lithium as well as cycle-lifecharacteristics consequently deteriorate. Accordingly, there are stilldemands to find a positive active material for a rechargeable lithiumbattery in which anisotropic volumetric expansion is minimized.

In order to improve the structural stability of the active materialduring charge and discharge, it is suggested that the Ni-based lithiatedoxide or the Co-based lithiated oxide be doped with other elements. InU.S. Pat No. 5,292,601, Li_(x)MO₂ (wherein M is an element selected fromCo, Ni, or Mn; x is 0.5-1) is suggested to improve the performance forLiCoO₂. However, there are continuing demands for further improvedpositive active materials, especially for structural stability andcycle-life characteristics.

SUMMARY OF THE INVENTION

In one embodiment, the present invention is directed to a positiveactive material for a rechargeable lithium battery comprising alithiated intercalation compound, and a coating layer formed on thelithiated intercalation compound. The coating layer includes asolid-solution compound and an oxide compound having at least twocoating elements. The oxide compound having at least two coatingelements is represented as the following Formula 1:M_(p)M′_(q)O_(r)  (1)wherein M and M′ are not the same and are each independently at leastone element selected from the group consisting of Zr, Al, Na, K, Mg, Ca,Sr, Ni, Co, Ti, Sn, Mn, Cr, Fe, and V,

0<p<1,

0<q<1, and

1<r≦2, r is determined based upon p and q. The solid-solution compoundis prepared by reacting the lithiated intercalation compound and theoxide compound. The coating layer has a fracture toughness of at least3.5 MPam^(1/2).

In another embodiment, the invention is directed to a positive activematerial for a rechargeable lithium battery comprising a lithiatedintercalation compound and a coating layer formed on the lithiatedintercalation compound. The coating layer comprises an oxide compoundhaving at least two coating elements represented by the followingFormula 1:M_(p)M′_(q)O_(r)  (1)wherein M and M′ are not the same and are each independently at leastone element selected from the group consisting of Zr, Al, Na, K, Mg, Ca,Sr, Ni, Co, Ti, Sn, Mn, Cr, Fe, and V;

0<p<1;

0<q<1; and

1<r≦2, where r is determined based upon p and q.

The present invention also provides a method of preparing a positiveactive material for a rechargeable lithium battery. In this method, acoating liquid comprising at least two coating elements is prepared. Tothe coating liquid, a lithiated intercalation compound is added andcoated. Then, the coated lithiated intercalation compound is subjectedto heat-treatment to provide a positive active material in which thelithiated intercalation compound with a coating layer has a fracturetoughness of at least 3.5 MPam^(1/2). The coating layer includes asolid-solution compound and an oxide compound having at least twocoating elements. The solid-solution compound is prepared by reactingthe lithiated intercalation compound and the oxide compound. The oxidecompound having at least two coating elements is represented as thefollowing Formula 1:M_(p)M′_(q)O_(r)  (1)wherein M and M′ are not the same and are each independently at leastone element selected from the group consisting of Zr, Al, Na, K, Mg, Ca,Sr, Ni, Co, Ti, Sn, Mn, Cr, Fe, and V; 0<p<1; 0<q<1; and 1<r≦2, where ris determined based upon p and q.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention, and many of the attendantadvantages thereof, will be readily apparent as the same becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings, wherein:

FIG. 1 is a view showing a concentration distribution of coatingelements from the surface to the center of the positive active materialfabricated by the method according to Example 1 of the presentinvention;

FIG. 2 is a graph showing charge-discharge characteristics at a 0.1 Crate of coin cells according to Example 1 of the present invention andComparative Examples 3 to 5;

FIG. 3 is a graph showing cycle-life characteristics of coin cellsaccording to Example 1 of the present invention and Comparative Examples1 to 5; and

FIG. 4 is a graph showing charge-discharge characteristics at theovervoltage of 4.6V of coin cells according to Example 1 of the presentinvention and Comparative Example 6.

FIG. 5 is a perspective view of a battery according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention is described in detail.

The present invention relates to a positive active material for arechargeable lithium battery having a lithiated intercalation compoundwith a surface on which a coating layer is formed. The coating layer hasa fracture toughness of at least 3.5 MPam^(1/2) and includes an oxidecompound having at least two coating elements and a solid-solutioncompound. The solid-solution compound is formed by reacting thelithiated intercalation compound with the oxide compound.

The lithiated intercalation compound is anisotropically expanded andcontracted during the intercalation/deintercalation reaction of lithiumions, and it thereby undergoes phase transition at the a-axis and c-axisof the positive active material. If the ratios of the volumetricexpansion and contraction of the lithiated intercalation compound areover 0.2%, too many micro-cracks are generated to ensure stability ofthe structure.

The present invention provides a coating layer on the surface of thelithiated intercalation compound in order to improve the structuralstability of the compound. The coating layer should be able to toleratethe anisotropic volumetric variation, and it includes a solid-solutioncompound and an oxide compound having at least two coating elements. Thesolid-solution compound is prepared by reacting the lithiatedintercalation compound and the oxide compound. The oxide compound havingat least two coating elements is represented by the following Formula 1:M_(p)M′_(q)O_(r)  (1)wherein M and M′ are not the same and are each independently at leastone element selected from the group consisting of Zr, Al, Na, K, Mg, Ca,Sr, Ni, Co, Ti, Sn, Mn, Cr, Fe, and V; 0<p<1; 0<q<1; and 1<r≦2, where ris determined based upon the values of p and q.

According to a preferred example of the present invention, the oxidecompound having at least two coating elements is a zirconium-containingoxide compound represented by the following Formula 2:Zr_(p)M′_(q)O_(r)  (2)wherein M′ is at least one element selected from the group consisting ofAl, Na, K, Mg, Ca, Sr, Ni, Co, Ti, Sn, Mn, Cr, Fe, and V, preferably Al;0<p<1; 0<q<1; and 1<r≦2, where r is determined based upon the values ofp and q.

In this embodiment, the positive active material of the presentinvention comprises a lithiated intercalation compound, and a coatinglayer is formed on the lithiated intercalation compound. The coatinglayer has a fracture toughness of at least 3.5 MPam^(1/2) and includes asolid-solution compound and an oxide compound having azirconium-containing oxide compound as shown in the above Formula 2. Thesolid-solution compound is prepared by reacting the lithiatedintercalation compound with the zirconium-containing oxide compound.

The coating layer has a fracture toughness of at least 3.5 MPam^(1/2)and preferably at least 10 MPam^(1/2). When the fracture toughness isless than 3.5 MPam^(1/2), the structural stability is not sufficientlyimproved.

The content of the coating element present in the coating layerpreferably ranges from 0.1 to 10 wt %, more preferably from 1 to 7 wt %.If the coating element is present in an amount less than 0.1 wt % in thecoating layer, the coating effect is not sufficient, while if thecoating element is present in an amount more than 10 wt % in the coatinglayer, it is also not desirable, since charge-discharge capacity andefficiency deteriorate.

The fracture toughness is a maximum point tolerable to mechanicalfracture, so that it is understood that the higher the fracturetoughness is, the more stable the material structure is. The fracturetoughness is generally measured by a single-edge-notched beam (SENB)method or an indentation crack length (ICL) method. Table 1 shows theresults of fracture toughness measurements made by the indentation cracklength (ICL) method.

TABLE 1 Oxide Compound Fracture Toughness (MPam^(1/2)) ZrAlO₄ 10˜14 ZrO₂ 8˜10 Al₂O₃ 2.7˜4.2 TiO₂ 2.38 B₂O₃ 1.44 SiO₂ 0.70

As shown in Table 1, the ternary-element oxide compound ZrAlO₄ (which isstoichiometrically equivalent to Zr_(0.5)Al_(0.5)O₂) has a fracturetoughness superior to that of the binary-element oxide compound.Notably, it was discovered that the structural stability of the positiveactive material improves and the cycle-life characteristics dramaticallyimprove when the lithiated intercalation compound is coated with acoating layer of a ternary-element oxide compound or an oxide compoundof more than 3 elements having a high fracture toughness. It ispostulated that the coating layer reduces the anisotropic volumetricvariation caused by intercalation/deintercalation of lithium ions duringcharge and discharge.

The coating layer includes, in addition to the oxide compound having atleast two coating elements, a solid-solution compound prepared byreacting the oxide compound with the lithiated intercalation compound.The solid-solution compound can be formed to the depth of no more than20,000 Å from the surface of the coating layer. The solid-solutioncompound is distributed with a concentration gradient such that theconcentration decreases toward the center of the positive activematerial.

Generally, the fracture toughness increases with an increase in thethickness of the coating layer, but the thickness preferably ranges from0.01 to 2 μm and more preferably from 0.01 to 0.1 μm. If the thicknessof the coating layer is less than 0.01 μm, the cycle-lifecharacteristics do not sufficiently improve, while if the thickness isover 2 μm, the capacity is reduced.

The lithiated intercalation compound is capable of intercalating lithiumions and includes a lithiated metal oxide or a lithiated calcogenidecompound. These compounds have a fundamentally cubic, hexagonal, ormonoclinic structure. Preferably the lithiated intercalation compound isselected from the following compounds, represented by Formulas (3) to(15):Li_(x)Mn_(1-y)M′_(y)A₂  (3)Li_(x)Mn_(1-y)M′_(y)O_(2-z)X_(z)  (4)Li_(x)Mn₂O_(4-z)X_(z)  (5)Li_(x)Mn_(2-y)M′_(y)A₄  (6)Li_(x)Co_(1-y)M′_(y)A₂  (7)Li_(x)Co_(1-y)M′_(y)O_(2-z)X_(z)  (8)Li_(x)Ni_(1-y)M′_(y)A₂  (9)Li_(x)Ni_(1-y)M′_(y)O_(2-z)X_(z)  (10)Li_(x)Ni_(1-y)Co_(y)O_(2-z)X_(z)  (11)Li_(x)Ni_(1-y-z)Co_(y)M′_(z)A_(α)  (12)Li_(x)Ni_(1-y-z)Co_(y)M′_(z)O_(2-α)X_(α)  (13)Li_(x)Ni_(1-y-z)Mn_(y)M′_(z)Aα  (14)Li_(x)Ni_(1-y-z)Mn_(y)M′_(z)O_(2-α)X_(α)  (15)wherein

0.95≦x≦1.1; 0≦y≦0.5; 0≦z≦0.5; 0≦α≦2;

M′ is an element selected from the group consisting of Al, Ni, Co, Mn,Cr, Fe, Mg, Sr, V, Sc, Y, and elements of the lanthanoid group;

A is an element selected from the group consisting of O, F, S, and P;and

X is an element selected from the group consisting of F, S, and P.

The particle size of the lithiated intercalation compound preferablyranges from 1 to 20 μm, and more preferably from 3 to 15 μm.

To coat the positive active material of the present invention, thecoating liquid having at least two coating elements is used. The coatingliquid is prepared by adding a coating-element source such as acoating-element-containing alkoxide, a coating-element-containing salt,or a coating-element-containing oxide to an organic solvent. Examples ofthe organic solvent include, but are not limited to, alcohols such asmethanol, ethanol, and isopropanol; hexane; chloroform; tetrahydrofuran;ether; methylene chloride; and acetone. The coating liquid is preparedby adding a coating-element source to one or more solvents to form asolution or a suspension.

The concentration of the coating-element is preferably 10 to 70 wt % ofthe coating liquid. When the concentration of the coating element isbelow 10 wt %, coating is not fully effective, whereas when theconcentration is more than 70 wt %, charge-discharge capacity andefficiency undesireably deteriorate.

Subsequently, the lithiated intercalation compound is introduced intothe coating liquid. The coating method preferably comprises a dipcoating method since it is a simple method, but it may include othercoating methods such as a spray coating method.

The coated lithiated intercalation compound is subjected toheat-treatment to provide a coated positive active material. The heattreatment process is preferably performed at a temperature ranging from300 to 800° C. for 3 to 10 hours. An additional drying step may be carryout before the heat treatment, preferably at a temperature ranging from80 to 200° C. for 1 to 5 hours. The lithiated intercalation compoundreacts with the oxide compound during the heat-treatment to provide asolid-solution compound. If the heat-treating temperature is below 300°C., the capacity and cycle-life characteristics do not improve, while ifit is over 800° C., the coating layer is burned out.

The resultant positive active material is added to and mixed with anorganic solvent together with a binder and a conductive agent to providea positive active material slurry. The slurry is coated on a collectorto provide a positive electrode 3 for a rechargeable lithium battery.The prepared positive electrode 3, along with a negative active materialto form a negative electrode 4, a separator 2 and an electrolyte are putinto a case 1 to fabricate a rechargeable lithium cell. The negativeelectrode comprises a negative active material capable of reversiblyintercalating lithium ions. The electrolyte comprises a lithiatedcompound and an organic solvent. The negative active material and theelectrolyte include any conventional materials capable of being used inthe rechargeable lithium battery art.

The following examples illustrate the present invention in furtherdetail, but the present invention is not limited by these examples.

EXAMPLE 1

50 wt % of a zirconium ethylhexanoisopropoxide suspension and 50 wt % ofan aluminum ethylhexanoisopropoxide suspension in a volume ratio of 1:1were mixed to obtain a coating liquid. The obtained coating liquid andLiCoO₂ powder having an average particle size of 10 μm were mixed in aweight ratio of 50:50 in 50 g of isopropanol to coat the LiCoO₂ powderwith the coating liquid. The coated LiCoO₂ powder was dried at 100° C.for 2 hours, then heat-treated at 400° C. for 10 hours to formsolid-solution compounds of ZrAlO₄ and LiCo_(1-a)Zr_(b)Al_(c)O₂(0<a≦0.6, 0<b≦0.2, 0<c≦0.2) on the surface thereof. Using the obtainedpositive active material precursor, a Super P conductive agent, and apolyvinylidene fluoride in a weight ratio of 92:4:4, a positive activematerial slurry was provided. The positive active material slurry filmwas cast on an Al-foil to about 100 μm, and then compressed to provide apositive electrode for a coin cell. The positive electrode was punchedin a circle shape with a diameter of 1.6 cm. Using the prepared positiveelectrode and a lithium counter-electrode, a coin cell was fabricated ina glove box. For the electrolyte, a 1 M LiPF₆ solution of ethylenecarbonate and dimethyl carbonate (1:1 volume ratio) was used.

EXAMPLE 2

A coin cell was fabricated by the same procedure as in Example 1, exceptthat 50 wt % of a zirconium ethylhexanoisopropoxide suspension was mixedwith 50 wt % of a nickel ethylhexanoisopropoxide suspension in a volumeratio of 1:1 to provide the coating liquid.

EXAMPLE 3

A coin cell was fabricated by the same procedure as in Example 1, exceptthat 50 wt % of a zirconium ethylhexanoisopropoxide suspension was mixedwith 50 wt % of an aluminum ethylhexanoisopropoxide suspension and 50 wt% of a nickel ethylhexanoisopropoxide suspension in a volume ratio of1:1:1 to provide the coating liquid.

EXAMPLE 4

A coin cell was fabricated by the same procedure as in Example 1, exceptthat LiNiO₂ having an average particle size of 10 μm was used instead ofLiCoO₂.

EXAMPLE 5

A coin cell was fabricated by the same procedure as in Example 1, exceptthat LiMn₂O₄ having an average particle size of 13 μm was used insteadof LiCoO₂.

EXAMPLE 6

A coin-type half-cell was fabricated by the same procedure as in Example1, except that LiNi_(0.9)Co_(0.1)Sr_(0.002)O₂ having an average particlesize of 13 μm was used instead of LiCoO₂.

EXAMPLE 7

A coin cell was fabricated by the same procedure as in Example 1, exceptthat LiNi_(0.8)Mn_(0.2)O₂ having an average particle size of 10 μm wasused instead of LiCoO₂.

EXAMPLE 8

A coin cell was fabricated by the same procedure as in Example 1, exceptthat Li_(1.03)Ni_(0.69)Mn_(0.19)Co_(0.1)Al_(0.07)Mg_(0.07)O₂ having anaverage particle size of 13 μm was used instead of LiCoO₂.

COMPARATIVE EXAMPLE 1

A coin cell was fabricated by the same procedure as in Example 1, exceptthat an uncoated LiCoO₂ powder having an average particle size of 10 μmwas used.

COMPARATIVE EXAMPLE 2

A coin cell was fabricated by the same procedure as in Example 1, exceptthat 4 wt % of an aluminum ethylhexanoisopropoxide suspension was usedas the coating liquid source.

COMPARATIVE EXAMPLE 3

A coin cell was fabricated by the same procedure as in Example 1, exceptthat 4 wt % of a titanium ethylhexanoisopropoxide suspension was used asthe coating liquid source.

COMPARATIVE EXAMPLE 4

A coin cell was fabricated by the same procedure as in Example 1, exceptthat 4 wt % of a boron ethylhexanoisopropoxide suspension was used asthe coating liquid source.

COMPARATIVE EXAMPLE 5

A coin cell was fabricated by the same procedure as in Example 1, exceptthat 4 wt % of a silicon ethylhexanoisopropoxide suspension was used asthe coating liquid source.

COMPARATIVE EXAMPLE 6

A coin cell was fabricated by the same procedure as in Example 1, exceptthat 4 wt % of a zirconium ethylhexanoisopropoxide suspension was usedas the coating liquid source.

COMPARATIVE EXAMPLE 7

A coin cell was fabricated by the same procedure as in Example 1, exceptthat an uncoated LiNiO₂ powder having an average particle size of 10 μmwas used.

FIG. 1 shows the Auger Electron Spectroscopy Analysis results of thepositive active material fabricated by the method according toExample 1. It is recognized that the concentrations of zirconium andaluminum decrease toward the center of the positive active material.Accordingly, zirconium and aluminum present in the coating layer arepresent mostly around the surface of the positive active material.

FIG. 2 shows a graph illustrating charge-discharge characteristics at0.1C at the voltage range of 2.75 to 4.4 V for the test cell of Example1 of the present invention, and for Comparative Examples 3 to 5. Asshown in FIG. 2, the coin cell of Example 1 has improved dischargecharacteristics over those of Comparative Examples 3 to 5.

FIG. 3 shows the cycle-life characteristics of coin cells of Example 1of the present invention and Comparative Examples 1 to 5. The cycle-lifecharacteristics were measured at 0.5 C at a voltage range of 2.75 to 4.4V. As shown in FIG. 3, cycle-life characteristics of the coin cell ofExample 1 in which the positive active material is coated with theternary-element oxide compound of ZrAlO₄ are distinctly better thanthose of Comparative Examples 1 to 5 in which the positive activematerial is coated with the binary-element oxide compound, and they aresimilar to those of Comparative Example 6.

FIG. 4 shows the charge-discharge characteristics of coin cells ofExample 1 of the present invention and those of Comparative Example 6.The charge-discharge characteristics were measured during repeatedcharge and discharge at 0.1 C at an overvoltage of 4.6 V. As shown inFIG. 4, the coin cell of Example 1 in which the positive active materialis coated with the ternary-element oxide compound of ZrAlO₄ has improvedcharge-discharge characteristics over that of Comparative Example 6 inwhich the positive active material is coated with the binary-elementoxide compound. FIG. 4 also shows that the charge-dischargecharacteristic at the 30th cycle, that is the cycle-life characteristic,of Example 1 is superior to that of Comparative Example 6.

As mentioned above, the positive electrode for the rechargeable lithiumbattery of the present invention has a coating layer having a very highfracture toughness so that the lithiated intercalation compound has astable structure due to reduced volumetric expansion duringintercalating and deintercalating lithium ions. Accordingly, thecycle-life and charge-discharge characteristics are significantlyimproved when the rechargeable lithium battery employs the positiveactive material of the present invention.

While the present invention has been described in detail with referenceto the preferred embodiments, those skilled in the art will appreciatethat various modifications and substitutions can be made thereto withoutdeparting from the spirit and scope of the present invention as setforth in the appended claims.

1. A positive active material for a rechargeable lithium batterycomprising: a lithiated intercalation compound; and a coating layerformed on the lithiated intercalation compound, the coating layercomprising an oxide compound having at least two coating elementsrepresented by the following Formula 1:M_(p)M′_(q)O_(r)  (1) wherein M and M′ are not the same and are eachindependently at least one element selected from the group consisting ofZr, Al, Na, K, Mg, Ca, Sr, Ni, Co, Ti, Sn, Mn, Cr, Fe, and V; 0<p<1;0<q<1; and 1<r≦2, where r is determined based upon p and q.
 2. Thepositive active material according to claim 1, wherein M is Zr.
 3. Thepositive active material according to claim 1, wherein the coating layerhas a fracture toughness of at least 10 Mpam^(1/2).
 4. The positiveactive material according to claim 1, wherein the lithiatedintercalation compound has a fundamental structure selected from thegroup consisting of cubic structures, hexagonal structures, andmonoclinic structures.
 5. The positive active material according toclaim 1, wherein the lithiated intercalation compound is selected fromthe group consisting of the following compounds represented by Formulas(3) to (15):Li_(x)Mn_(1-y)M′_(y)A₂  (3)Li_(x)Mn_(1-y)M′_(y)O_(2-z)X_(z)  (4)Li_(x)Mn₂O_(4-z)X_(z)  (5)Li_(x)Mn_(2-y)M′_(y)A₄  (6)Li_(x)Co_(1-y)M′_(y)A₂  (7)Li_(x)Co_(1-y)M′_(y)O_(2-z)X_(z)  (8)Li_(x)Ni_(1-y)M′_(y)A₂  (9)Li_(x)Ni_(1-y)M′_(y)O_(2-z)X_(z)  (10)Li_(x)Ni_(1-y)Co_(y)O_(2-z)X_(z)  (11)Li_(x)Ni_(1-y-z)Co_(y)M′_(z)A_(α)  (12)Li_(x)Ni_(1-y-z)Co_(y)M′_(z)O_(2-α)X_(α)  (13) Li_(x)Ni_(1-y-z)Mn_(y)M′_(z)Aα  (14)Li_(x)Ni_(1-y-z)Mn_(y)M′_(z)O_(2-α)X_(α)  (15) wherein 0.95≦x1.1;0≦y≦0.5; 0≦z≦0.5; 0≦α≦2; M′ is an element selected from the groupconsisting of Al, Ni, Go, Mn, Cr, Fe, Mg, Sr, V, Sc, Y, and elements ofthe lanthanoid group; A is an element selected from the group consistingof O, F, S, and P; and X is an element selected from the groupconsisting of F, S, and P.
 6. The positive active material according toclaim 1, wherein the coating element has a concentration gradient inwhich the concentration gradually decreases from the surface toward thecenter of the positive active material.
 7. The positive active materialaccording to claim 1, wherein the coating layer has a thickness rangingfrom 0.01 to 2 μm.
 8. The positive active material according to claim 1,wherein the coating element is present in the coating layer in an amountranging from 0.1 to 10 wt %.
 9. A positive active material for arechargeable lithium battery comprising: a lithiated intercalationcompound; and a coating layer formed on the lithiated intercalationcompound, the coating layer comprising a solid-solution compound and anoxide compound having at least two coating elements represented by thefollowing Formula 1:M_(p)M′_(q)O_(r)  (1) wherein M and M′ are not the same and are eachindependently at least one element selected from the group consisting ofZr, Al, Na, K, Mg, Ca, Sr, Ni, Co, Ti, Sn, Mn, Cr, Fe, and V; 0<p<1;0<q<1; and 1<r≦2, where r is determined based upon p and q, wherein thesolid-solution compound is prepared by reacting the lithiatedintercalation compound with the oxide compound, and the coating layerhas a fracture toughness of at least 3.5 MPam^(1/2).
 10. The positiveactive material according to claim 9, wherein M is Zr.
 11. The positiveactive material according to claim 9, wherein the coating layer has afracture toughness of at least 10 Mpam^(1/2).
 12. The positive activematerial according to claim 9, wherein the lithiated intercalationcompound has a fundamental structure selected from the group consistingof cubic structures, hexagonal structures, and monoclinic structures.13. The positive active material according to claim 9, wherein thelithiated intercalation compound is selected from the group consistingof the following compounds represented by Formulas (3) to (15):Li_(x)Mn_(1-y)M′_(y)A₂  (3)Li_(x)Mn_(1-y)M′_(y)O_(2-z)X_(z)  (4)Li_(x)Mn₂O_(4-z)X_(z)  (5)Li_(x)Mn_(2-y)M′_(y)A₄  (6)Li_(x)Co_(1-y)M′_(y)A₂  (7)Li_(x)Co_(1-y)M′_(y)O_(2-z)X_(z)  (8)Li_(x)Ni_(1-y)M′_(y)A₂  (9)Li_(x)Ni_(1-y)M′_(y)O_(2-z)X_(z)  (10)Li_(x)Ni_(1-y)Co_(y)O_(2-z)X_(z)  (11)Li_(x)Ni_(1-y-z)Co_(y)M′_(z)A_(α)  (12)Li_(x)Ni_(1-y-z)Co_(y)M′_(z)O_(2-α)X_(α)  (13)Li_(x)Ni_(1-y-z)Mn_(y)M′_(z)Aα  (14)Li_(x)Ni_(1-y-z)Mn_(y)M′_(z)O_(2-α)X_(α)  (15) wherein 0.95≦x≦1.1;0≦y≦0.5; 0≦z≦0.5; 0≦α≦2; M′ is an element selected from the groupconsisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, Sc, Y, and elements ofthe lanthanoid group; A is an element selected from the group consistingof O, F, S, and P; and X is an element selected from the groupconsisting of F, S, and P.
 14. The positive active material according toclaim 9, wherein the coating element has a concentration gradient inwhich the concentration gradually decreases from the surface toward thecenter of the positive active material.
 15. The positive active materialaccording to claim 9, wherein the coating layer has a thickness rangingfrom 0.01 to 2 μm.
 16. The positive active material according to claim9, wherein the coating element is present in the coating layer in anamount ranging from 0.1 to 10 wt %.
 17. A positive active material for arechargeable lithium battery comprising: a lithiated intercalationcompound; and a coating layer formed on the lithiated intercalationcompound, the coating layer comprising a solid-solution compound and aZr-containing oxide compound of Zr_(0.5)Al_(0.5)O₂, wherein thesolid-solution compound is prepared by reacting the lithiatedintercalation compound with the Zr-containing compound, and the coatinglayer has a fracture toughness of at least 10 MPam^(1/2).
 18. A methodof preparing a positive active material for a rechargeable lithiumbattery comprising: preparing a coating liquid comprising an oxidecompound of at least two coating elements; adding a lithiatedintercalation compound to the coating liquid and coating the lithiatedintercalation compound with the coating liquid; and heat-treating thecoated lithiated intercalation compound, wherein the surface oflithiated intercalation compound is provided with a coating layercomprising a solid-solution compound and the oxide compound representedby the following Formula 1:M_(p)M′_(q)O_(r)  (1) wherein M and M′ are not the same and are eachindependently at least one element selected from the group consisting ofZr, Al, Na, K, Mg, Ca, Sr, Ni, Co, Ti, Sn, Mn, Cr, Fe, and V; 0<p<1;0<q<1; and 1<r≦2, where r is determined based upon p and q, wherein thesolid-solution compound is prepared by reacting the lithiatedintercalation compound with the oxide compound, and the coating layerhas a fracture toughness of at least 3.5 MPam^(1/2).
 19. The methodaccording to claim 18, wherein M is Zr.
 20. The method according toclaim 18, wherein the oxide compound is Zr_(0.5)Al_(0.5)O₂.
 21. Themethod according to claim 18, wherein the lithiated intercalationcompound has a fundamental structure selected from the group consistingof cubic structures, hexagonal structures, and monoclimc structures. 22.The method according to claim 18, wherein the lithiated intercalationcompound is selected from the group consisting of the followingcompounds represented by Formulas (3) to (15):Li_(x)Mn_(1-y)M′_(y)A₂  (3)Li_(x)Mn_(1-y)M′_(y)O_(2-z)X_(z)  (4)  Li_(x)Mn₂O_(4-z)X_(z)  (5)Li_(x)Mn_(2-y)M′_(y)A₄  (6)Li_(x)Co_(1-y)M′_(y)A₂  (7)Li_(x)Co_(1-y)M′_(y)O_(2-z)X_(z)  (8)Li_(x)Ni_(1-y)M′_(y)A₂  (9)Li_(x)Ni_(1-y)M′_(y)O_(2-z)X_(z)  (10)Li_(x)Ni_(1-y)Co_(y)O_(2-z)X_(z)  (11)Li_(x)Ni_(1-y-z)Co_(y)M′_(z)A_(α)  (12)Li_(x)Ni_(1-y-z)Co_(y)M′_(z)O_(2-α)X_(α)  (13)Li_(x)Ni_(1-y-z)Mn_(y)M′_(z)Aα  (14)Li_(x)Ni_(1-y-z)Mn_(y)M′_(z)O_(2-α)X_(α)  (15) wherein 0.95≦x≦1.1;0≦y≦0.5; 0≦z≦0.5; 0≦α≦2; M′ is an element selected from the groupconsisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, Sc, Y, and elements ofthe lanthanoid group; A is an element selected from the group consistingof O, F, S, and P; and X is an element selected from the groupconsisting of F, S, and P.
 23. The method according to claim 18, whereinthe coating layer has a thickness ranging from 0.01 to 2 μm.
 24. Themethod according to claim 18, wherein the coating element is present inthe coating layer in an amount ranging from 10 to 70 wt %.
 25. Themethod according to claim 18, wherein the coating element has aconcentration gradient in which the concentration gradually decreasesfrom the surface toward the center of the positive active material. 26.The method according to claim 18, wherein the heat-treatment step isperformed at a temperature ranging from 300 to 800°.
 27. The methodaccording to claim 18, wherein the heat-treatment step is performed fora time ranging from 3 to 10 hours.
 28. A rechargeable lithium batterycomprising a positive active material comprising: a lithiatedintercalation compound; and a coating layer formed on the lithiatedintercalation compound, the coating layer comprising a solid-solutioncompound and an oxide compound having at least two coating elementsrepresented by the following Formula 1:M_(p)M′_(q)O_(r)  (1) wherein M and M′ are not the same and are eachindependently at least one element selected from the group consisting ofZr, Al, Na, K, Mg, Ca, Sr, Ni, Co, Ti, Sn, Mn, Cr, Fe, and V; 0<p<1;0<q<1; and 1<r≦2, where r is determined based upon p and q, wherein thesolid-solution compound is prepared by reacting the lithiatedintercalation compound with the oxide compound, and the coating layerhas a fracture toughness of at least 3.5 MPam^(1/2).